The Clp protease system is an important element in protein metabolism in both bacteria and plant plastids. It consists of the ClpP protease, which by itself is probably inactive on most physiological substrates, and the ClpA family of ATPases, which present substrates to CIpP. The CIp system plays a major role ATP-dependent turnover of proteins in bacteria and plants, but may also play a role in facilitating the transport, folding, and activation of some proteins. This system may, therefore, repre sent an important decision point in the cellular pathway of folding and degradation. We are studying ClpP and three members of the ClpA family of ATPases from E. coli (ClpA, ClpB, and ClpX) using wild type and engineered proteins expressed from cloned genes. Previously we demonstrated that E. coli ClpP is a tetradecamer of 21.5kDa subunits'. Recently, we have grown crystals of ClpP that diffract to 2.2A resolution. We propose to determine its three-dimensional structure. A high resolution str ucture of ClpP may provide important clues into: a) the details of mechanism of action of serine proteases, b) clues to the nature of the allosteric regulation of ClpP, and c) aid in understanding the requirement of ATP in the mechanism of the CIp protease.